Derivative Instruments, Hedging Activities and Fair Value Measurements (Details) bbl in Millions, $ in Millions, ft³ in Billions		12 Months Ended
		Dec. 31, 2018 USD ($) bbl ft³	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Derivative [Line Items]
Proceeds from the settlement of interest rate derivative instruments | $		$ 22.1	$ 30.6	$ 6.1
Gain on the sale of swaption | $		29.4
Carrying amount of hedged asset | $		50.2	84.0
Forward Starting Swaps [Member] | Derivatives in cash flow hedging relationships [Member]
Derivative [Line Items]
Notional amount of settled derivative instruments | $		275.0	275.0	250.0
Proceeds from the settlement of interest rate derivative instruments | $		$ 22.1	$ 30.6	$ 6.1
Designated as Hedging Instrument [Member] | Natural gas processing: Forecasted natural gas purchases for plant thermal reduction (PTR) [Member] | Derivatives in cash flow hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume | ft³	[1],[2]	4.9
Designated as Hedging Instrument [Member] | Natural gas processing: Forecasted sales of NGLs [Member] | Derivatives in cash flow hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2]	1.0
Designated as Hedging Instrument [Member] | Octane enhancement: Forecasted purchases of NGLs [Member] | Derivatives in cash flow hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2]	1.8
Designated as Hedging Instrument [Member] | Octane enhancement: Forecasted sales of octane enhancement products [Member] | Derivatives in cash flow hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2]	3.1
Derivative instruments, long-term volume	[1],[2]	0.1
Designated as Hedging Instrument [Member] | Natural gas marketing: Natural gas storage inventory management activities [Member] | Derivatives in fair value hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume | ft³	[1],[2]	3.3
Designated as Hedging Instrument [Member] | NGL marketing: Forecasted purchases of NGLs and related hydrocarbon products [Member] | Derivatives in cash flow hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2]	33.6
Derivative instruments, long-term volume	[1],[2]	4.3
Designated as Hedging Instrument [Member] | NGL marketing: Forecasted sales of NGLs and related hydrocarbon products [Member] | Derivatives in cash flow hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2]	45.0
Derivative instruments, long-term volume	[1],[2]	1.7
Designated as Hedging Instrument [Member] | NGL marketing: NGLs inventory management activities [Member] | Derivatives in fair value hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2]	0.3
Designated as Hedging Instrument [Member] | Refined products marketing: Forecasted purchases of refined products [Member] | Derivatives in cash flow hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2]	1.0
Designated as Hedging Instrument [Member] | Refined products marketing: Forecasted sales of refined products [Member] | Derivatives in cash flow hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2]	2.0
Designated as Hedging Instrument [Member] | Refined products marketing: Refined products inventory management activities [Member] | Derivatives in fair value hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2]	0.5
Designated as Hedging Instrument [Member] | Crude oil marketing: Forecasted purchases of crude oil [Member] | Derivatives in cash flow hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2]	18.4
Derivative instruments, long-term volume	[1],[2]	1.9
Designated as Hedging Instrument [Member] | Crude oil marketing: Forecasted sales of crude oil [Member] | Derivatives in cash flow hedging relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2]	28.5
Derivative instruments, long-term volume	[1],[2]	1.9
Not Designated as Hedging Instrument [Member] | Natural gas risk management activities [Member] | Derivatives in mark-to-market relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume | ft³	[1],[2],[3],[4]	77.5
Derivative instruments, long-term volume | ft³	[1],[2],[3],[4]	0.9
Current natural gas hedging volumes designated as an index plus or minus a discount | ft³		29.8
Not Designated as Hedging Instrument [Member] | NGL risk management activities [Member] | Derivatives in mark-to-market relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2],[4]	3.3
Not Designated as Hedging Instrument [Member] | Refined products risk management activities [Member] | Derivatives in mark-to-market relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2],[4]	2.6
Not Designated as Hedging Instrument [Member] | Crude oil risk management activities [Member] | Derivatives in mark-to-market relationships [Member]
Derivative [Line Items]
Derivative instruments, current volume	[1],[2],[4]	26.3
Derivative instruments, long-term volume	[1],[2],[4]	3.2

[1] The maximum term for derivatives designated as cash flow hedges, derivatives designated as fair value hedges and derivatives not designated as hedging instruments is December 2020, June 2019 and December 2020, respectively.

[2] Volume for derivatives designated as hedging instruments reflects the total amount of volumes hedged whereas volume for derivatives not designated as hedging instruments reflects the absolute value of derivative notional volumes.

[3] Current volumes include 29.8 Bcf of physical derivative instruments that are predominantly priced at a marked-based index plus a premium or minus a discount related to location differences.

[4] Reflects the use of derivative instruments to manage risks associated with transportation, processing and storage assets.